Bi-fuel Engine Using Hydrogen

ABSTRACT

A method is disclosed for making a transition from fueling an engine with hydrogen to another fuel. That other fuel may be gasoline, a gasoline and alcohol mixture, or gaseous fuels, as examples. The other fuel has the capability of providing higher BMEP than the hydrogen because of better air utilization and because the other fuel occupies less volume of the combustion chamber. Because a desirable equivalence ratio to burn hydrogen is at 0.5 or less and a desirable equivalence ratio to burn other fuel is at 1.0, when a demand for BMEP that leads to a transition change from hydrogen fuel to the other fuel, the amount of air supplied to the engine is decreased to provide more torque and vice versa. During a transition in which liquid fuel supply is initiated, it may desirable to continue to provide some hydrogen, not leaner than 0.1 hydrogen equivalence ratio.

FIELD OF THE INVENTION

A method to operate an internal combustion engine which is supplied withboth hydrogen fuel and another fuel is disclosed.

BACKGROUND

Because of concerns about greenhouse gases that are emitted fromcarbon-containing fuels, such as gasoline, diesel, and alcohol fuels,there is keen interest in fueling motor vehicles with hydrogen, whichproduces water upon combustion. Hydrogen-fueled internal-combustionengines suffer from a low power output compared to gasoline or dieselpowered engines due to hydrogen being a gaseous fuel which takes up muchof the volume in the cylinder, particularly when compared to dense fuelslike gasoline or diesel fuel. Furthermore, hydrogen combustion islimited to operating at an equivalence ratio of about 0.5 or less due toincreasing combustion harshness and, if it is a concern, rapidlyincreasing NOx emission. An equivalence ratio of one is a stoichiometricratio meaning that the proportion of fuel to air is such that all theoxygen and fuel could burn completely. An equivalence ratio of 0.5 is alean ratio in which the amount of air supplied is double that needed tocompletely consume the fuel. Such a limit in equivalence ratio resultsin about half the fuel delivery as could be consumed by the amount ofair in the chamber, and consequently about half of the torque developedby the engine than if at a stoichiometric proportion.

Equivalence ratio is defined as the mixture's fuel to air ratio (bymass) divided by the fuel to air ratio for a stoichiometric mixture. Astoichiometric mixture has an equivalence ratio of 1.0; lean mixturesare less than 1.0; and, rich mixtures are greater than 1.0.

SUMMARY OF THE INVENTION

The inventors of the present invention have recognized that by operatingon two fuels: hydrogen and gasoline, as an example, the engine could beoperated on hydrogen at low torque levels and on gasoline at highertorque levels. Hydrogen combusts readily at very lean equivalence ratiosand is well suited to burning robustly at very low torques with at most,a minimum of throttling. Gasoline is well suited to providing hightorque because of its high energy density and ability to operate atstoichiometric. The inventors of the present invention propose a bifuelengine in which transitions are made between operating on hydrogen andanother fuel.

The high torque fuel can be a hydrocarbon, such as natural gas, propane,gasoline, or alcohols, such as methanol or ethanol. Furthermore,combinations of the gaseous fuel or combinations of the liquid fuels mayalso be used, such as E85, a mixture of 85% ethanol with 15% gasoline.High torque fuels contain carbon, which upon combustion reacts to formcarbon dioxide, a greenhouse gas. Because hydrogen produces only wateras the product of combustion, it does not form a greenhouse gas. Thus,it is desirable to operate on hydrogen when possible and using thecarbon containing fuels as needed to provide the desired torque.

A normalized engine torque commonly used by one skilled in the art isBMEP, brake mean effective pressure, which for 4-stroke engines is2*P/(V*N), where P is brake power, V is displaced volume, and N isengine rpm.

A method for making a transition from a first to a second operating modeis disclosed in which the air supply is decreased, supply of a firstfuel is decreased, and supply of a second fuel is initiated at the startof the transition. The first fuel is substantially 100% hydrogen and thesecond fuel is primarily comprised of hydrocarbons, gasoline or gasolineand alcohol mixtures, as examples. Alternatively, the second fuel is agaseous hydrocarbon. During the transition, the amount of hydrogen iscontinuously decreased s that at termination of the transition, hydrogenis no longer being supplied to the engine. Concurrently, the amount ofthe second fuel is increased during the transition in coordination withthe decrease of hydrogen. The transition is initiated when a demand fortorque causes the equivalence ratio of hydrogen fuel to exceed athreshold, which threshold is approximately 0.5. The air supply decreaseis accomplished by closing the engine's throttle valve with the airsupply decrease being in the range of 30-60% during the transition. Inone embodiment, the transition is further initiated in response to theengine piston speed exceeding a threshold. Engine piston speed iscomputed as 2*S*N, where S is stroke and N is engine rpm. The pistonspeed is not constant through the revolution; the piston speed computedhere is average piston speed.

Also disclosed is a method to transition between two operating modes inan internal combustion engine in which air supply is increasedsubstantially, supply of hydrogen is initiated and supply of a secondfuel is decreased, all occurring roughly at the initiation of thetransition. The transition is initiated in response to a demand for atorque decrease below a threshold BMEP: that BMEP being 3.5 to 5 bar fora naturally aspirated engine and between 6 and 8 bar for a pressurecharged engine. During the transition, air supply increases in the rangeof 30-60%. The supply of hydrogen to the engine upon transitioninitiation causes the equivalence ratio with respect to only thehydrogen fuel to be at least 0.1.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment in which the invention is used to advantage,referred to herein as the Detailed Description, with reference to thedrawings, wherein:

FIG. 1 is a schematic of an engine having two fuel supplies;

FIGS. 2 a-b show engine operating maps of BMEP and piston speed, showingoperating zones for two fuels;

FIG. 3 shows an engine operating map of BMEP and catalyst temperature,showing operating zones for two fuels; and

FIGS. 4 and 5 show timelines of transitions from hydrogen to gasoline.

DETAILED DESCRIPTION

A 4-cylinder internal combustion engine 10 is shown, by way of example,in FIG. 1. Engine 10 is supplied air through intake manifold 12 anddischarges spent gases through exhaust manifold 14. An intake ductupstream of the intake manifold 12 contains a throttle valve 32 which,when actuated, controls the amount of airflow to engine 10. Sensors 34and 36 installed in intake manifold 12 measure air temperature and massair flow (MAF), respectively. Sensor 31, located in intake manifold 14downstream of throttle valve 32, is a manifold absolute pressure (MAP)sensor. A partially closed throttle valve 32 causes a pressuredepression in intake manifold 12 compared to the pressure on theupstream side of throttle valve 32. When a pressure depression exists inintake manifold 12, exhaust gases are caused to flow through exhaust gasrecirculation (EGR) duct 19, which connects exhaust manifold 14 tointake manifold 12. Within EGR duct 19 is EGR valve 18, which isactuated to control EGR flow. Hydrogen fuel is supplied to engine 10 byfuel injectors 30, injecting directly into cylinders 16, and portinjectors 26 injecting a liquid fuel into intake manifold 12. Thisarrangement is shown by way of example and is not intended to belimiting. In other embodiments include having port injectors 26supplying hydrogen fuel and direct injectors 30 supplying liquid fuel.Alternatively, both fuels are supplied through direct fuel injectors. Inyet another embodiment both fuels are supplied by port injectors. Thefuel other than hydrogen, in another embodiment, is a gaseoushydrocarbon fuel such as methane. Each cylinder 16 of engine 10 containsa spark plug 28. The crankshaft (not shown) of engine 10 is coupled to atoothed wheel 20. Sensor 22, placed proximately to toothed wheel 20,detects engine 10 rotation. Other methods for detecting crankshaftposition may alternatively be employed.

In one embodiment, the engine is pressure charged by a compressor 58 inthe engine intake. By increasing the density of air supplied to engine10, more fuel can be supplied at the same equivalence ratio. By doingso, engine 10 develops more power. Compressor 58 can be a superchargerwhich is typically driven off the engine. Alternatively, compressor 58is connected via a shaft with a turbine 56 disposed in the engineexhaust. Turbine 56, as shown in FIG. 1, is a variable geometry turbine;but, may be, in an alternative embodiment, a non-variable device. Inanother embodiment, the engine is naturally aspirated, in whichembodiment elements 56 and 58 are omitted. Downstream of turbine 56 isthree-way catalyst 66. Three-way catalyst 66 can alternatively be placeupstream of turbine 56 for faster light-off. Alternatively, catalyst 66is a lean NOx trap or lean NOx catalyst having the capability to reduceNOx at a lean equivalence ratio.

Two fuel tanks, 60 and 64, supply the two fuels. In the embodiment shownin FIG. 1, tank 60 contains liquid fuel and tank 64 contains hydrogen.However, as described above, the inventors of the present inventioncontemplate a variety of possible fuel combinations, with theappropriate fuel storage container included. In tank fuel pump 62pressurizes liquid fuel. Fuel tank 64 is under high pressure. Typically,no pressurization is required, but a pressure regulator may be used.

It is known in the prior art to make transitions between engineoperating modes. For example, in stratified charge gasoline engines,transitions between lean, stratified to premixed, stoichiometricoperation are known to pose a challenge because the equivalence ratiochanges from lean to rich abruptly, with the fuel remaining constant. Inthe present invention, the equivalence ratio also changes abruptly whenswitching fuels because the best combination of hydrogen operatingcharacteristics are achieved at an equivalence ratio less than 0.5;whereas, desirable fuel and emission operating characteristics areachieved with other fuels (hydrocarbons, alcohols, etc.) at anequivalence ratio of 1.0. Fuel transitions can be accomplished in asingle cycle, whereas air lags thereby causing challenges during thetransitions. The present invention differs from prior art transitions instratified charge engines because in the present invention the fuelchanges as well as the equivalence ratio.

It is known in the prior art to operate bi-fuel engines in whichtransitions are made between two fuels, such as between gasoline andpropane or between gasoline and ethanol. However, most known fuels(gaseous hydrocarbons, liquid hydrocarbons, and alcohols) have a narrowrange of flammability, equivalence ratio (roughly 0.65 lean limit and1.7 rich limit) compared with hydrogen fuel (roughly 0.10 lean limit and3 rich limit). Because most fuels cannot combust robustly at very leanequivalence ratios, their stable, lean operation occurs in a region inwhich high NOx is produced. Thus, most fuels, except hydrogen, areoperated at stoichiometric, i.e., equivalence ratio of 1. Because verylean mixtures of hydrogen combust robustly, the amount of NOx producedis small allowing such lean operation without a great emission concern.Even though hydrogen can be combusted in a wide range of equivalenceratios, in an internal combustion engine, it is used in the 0.15 to 0.5equivalence ratio range because when operating richer than 0.5equivalence ratio harsh combustion and autoignition of the hydrogenresults, conditions which are to be avoided. Thus, a bi-fuel engine, inwhich one of the two fuels is hydrogen, when making a transition fromhydrogen to gasoline, a switch from an equivalence ratio of about 0.5,or leaner, to 1.0 occurs.

In summary, the present invention distinguishes between the prior arttransitions between stratified, lean operation and stoichiometricoperation, as discussed above, in that both a transition in equivalenceratio and fuel type occurs. The present invention distinguishes betweenthe prior art bi-fuel transition because when one of the fuels ishydrogen, according to the present invention, switching among combustionmodes results in an increase in both fuel type and equivalence ratio;whereas, in the prior art in which neither of the two fuels is hydrogen,the equivalence ratio does not substantially change when the fuel typechanges.

Gaseous fuels that are delivered by an electronic fuel injector can beturned on, off, or anywhere in between in a single cycle with the onlytransient issue being inventory of fuel in the intake manifold in thecase of the fuel injector being located in the intake port. Liquid fuelsthat are supplied directly to the combustion chamber (direct injected)can be affected in a single cycle. However, liquid fuels that aresupplied into the intake port (port injected) present some difficultiesdue to fuel films that form on port surfaces. That is, when activatinginjectors, some of the fuel sprayed wets manifold walls and does notenter the combustion chamber directly. When deactivating liquid, portinjectors, the fuel films on the walls remaining on intake port wallsare removed and are inducted into the combustion chamber; this fuelinventory takes several intake events to empty. For example, changingthe amount of air being inducted into a cylinder abruptly presents anissue as it takes several engine cycles for a manifold to fill or empty.Thus, the transition from one fuel to the other takes at least severalengine cycles. In one embodiment, a switch between fuels is accomplishedover tens of cycles.

In one embodiment, both fuels are delivered during the transition periodwhile the supplied air is adjusted to the new operating condition. It isknown to those skilled in the art that hydrogen, when used to supplementgasoline (or other hydrocarbon fuel) facilitates combustion at asubstantially leaner equivalence ratio than would be possible withgasoline alone.

In FIG. 2 a, it is shown the fuel 2 is used when the threshold BMEP isexceeded. This threshold is associated with an equivalence ratio of thehydrogen which is greater than a desirable level, e.g., 0.5. That is, toproduce more than the threshold BMEP, the hydrogen equivalence ratiowould exceed 0.5. In FIG. 2 b, an additional constraint is placed onhydrogen operation in that when the piston speed exceeds a certainthreshold, the engine transitions to fuel 2.

When cold, the engine starts on hydrogen fuel, which presents no coldstart vaporization and mixing issues such as a liquid fuel. In FIG. 3,fuel 2 is only used when both the catalyst has attained its light-offtemperature and the threshold BMEP has been exceeded.

In FIG. 4, one embodiment of a transition from hydrogen to gasoline isshown in a timeline. Before the transition, hydrogen is used; after thetransition, gasoline is used; and during the transition, a combinationof the two fuels is used. In the top graph, a, torque is increasing. Inthe bottom graph, e, the equivalence ratio, Φ, is less than 0.5 prior tothe transition. As discussed above, a transition from hydrogen togasoline is desirable when the hydrogen equivalence ratio approaches0.5; thus, the transition is initiated. In graph c, the amount ofhydrogen provided increases prior to the transition to provide theincreased torque of graph a. Prior to the transition, the air deliveryrate, dm_(a)/dt of graph b, remains constant with the additional torqueprovided by increasing hydrogen. At transition initiation, the throttleis partially closed and the amount of air is decreased. Air supplydecreases such that the air supplied by the end of the transition isthat required to provide Φ=1.0, which is the desired equivalence ratiofor all fuels, except hydrogen. One of the reasons that there is atransition period is that air delivery cannot be changed in one enginecycle. Instead, even when the throttle is opened rapidly, it takesseveral engine cycles for the manifold to fill and the desired amount ofair to be provided to the engine. Because the air is more than desiredright after the start of the transition, hydrogen supply is continued.It is known by those skilled in the art, that by supplementing aconventional fuel with hydrogen, that the conventional fuel can robustlycombust at an equivalence ratio at which it is unable to do so withoutthe presence of hydrogen. Thus, the hydrogen continues through thetransition period, until the equivalence ratio achieves the desired 1.0,at which time the hydrogen supply is discontinued. Alternatively, butnot shown in the Figure, the hydrogen supply could be discontinued whenthe equivalence ratio reaches a ratio that the conventional fuel, e.g.,gasoline, can robustly combust, such as greater than 0.8. Gasolinesupply is initiated at the start of the transition. However, asdiscussed above, because the air cannot be reduced as quickly asdesired, the hydrogen is continued into the transition period to ensurethe combustion. Through the transition period, the gasoline is increasedand the hydrogen decreased, as well as the air decreasing, so that bythe end of the transition period, the gasoline operation takes over withno hydrogen assistance.

In FIG. 5, an alternative embodiment is shown in which the initialportion of the transition is similar to that shown in FIG. 4. However,at a point during the transition, the equivalence ratio is bumped up to1.0 and maintained at 1.0 for the remainder of the transition. This isdone to avoid the high NOx region of 0.85-0.90 phi. However, during thistransition period of 1.0 equivalence ratio, the hydrogen supply iscontinuously being decreased and the gasoline supply is increased. Atthe end of the transition, hydrogen supply has ceased.

In the above discussion, a hydrogen-to-gasoline transition is described.However, the reference to gasoline is provided by way of example and isnot intended to be limiting. Furthermore, the transition occurring atΦ=0.5 is also by way of example. The actual transition may occur atslightly lower or higher equivalence ratios than exactly 0.5.

A transition from a higher torque to a lower torque in which gasoline(or other fuel) operation is transitioned to hydrogen operation can beaccomplished in the reverse of what is shown in FIGS. 4 and 5. If thefuel other than hydrogen is a liquid fuel and is port injected, theinventory of the fuel in the intake manifold is accounted for to providethe desired fuel into the combustion chamber.

While several modes for carrying out the invention have been describedin detail, those familiar with the art to which this invention relateswill recognize alternative designs and embodiments for practicing theinvention. The above-describe embodiments are intended to beillustrative of the invention, which may be modified within the scope ofthe following claims.

1. A method to transition from a first to a second operating mode in aninternal combustion engine, comprising: decreasing air supplysubstantially, said decrease starting at transition initiation;decreasing an amount of a first fuel supplied to the engine attransition initiation; initiating supply of a second fuel to the engineat transition initiation; and increasing supply of the second fuelabruptly in an amount to cause equivalence ratio to increase abruptly to1.0 wherein the increasing is performed in response to equivalence ratioexceeding a threshold equivalence ratio.
 2. The method of claim 1wherein said first fuel is substantially 100% hydrogen and said secondfuel is primarily comprised of hydrocarbons.
 3. The method of claim 1,further comprising: decreasing continually a supplied amount of saidfirst fuel during said transition wherein at termination of saidtransition said first fuel is no longer being supplied to the engine;and.
 4. The method of claim 1 wherein said second fuel is a liquid fuel.5. The method of claim 1 wherein said first fuel is hydrogen and saidtransition is initiated in response to a demand for a torque increasewhen said equivalence ratio is roughly 0.5.
 6. The method of claim 1wherein said threshold equivalence ratio is about 0.85.
 7. The method ofclaim 1 wherein said air supply is decreased by 30-60% during thetransition.
 8. The method of claim 1 wherein the transition is initiatedin response to engine piston speed exceeding a threshold.
 9. The methodof claim 1 wherein said transition is initiated in response to BMEPdemand exceeding a threshold.
 10. A method to transition between twooperating modes in an internal combustion engine, comprising: conductingthe transition in a transition initiation phase followed by a transitioncompletion phase wherein an equivalence ratio of 1.0 is maintainedduring the transition initiation phase and equivalence ratio decreasesduring the transition completion phase; the transition initiation phasecomprising: increasing air supply; initiating supply of a first fuel,hydrogen, to the engine; and decreasing supply of a second fuel to theengine; and the transition completion phase comprising furtherdecreasing supply of the second fuel abruptly in an amount to causeequivalence ratio to decrease abruptly to an equivalence ratio below athreshold equivalence ratio.
 11. The method of claim 10 wherein saidtransition is initiated in response to a demand for a torque decreasebelow a threshold BMEP. 12-13. (canceled)
 14. The method of claim 10wherein the equivalence ratio threshold is about 0.85.
 15. The method ofclaim 10 wherein said supply of hydrogen to the engine during thetransition termination phase causes the equivalence ratio with respectto only the hydrogen fuel to be at least 0.1.
 16. The method of claim 10wherein said second fuel is a liquid fuel.
 17. A method to transitionfrom a first to a second operating mode in an internal combustionengine, comprising: conducting the transition in a transition initiationphase followed by a transition completion phase with an equivalenceratio of 1.0 being maintained during the transition completion phase andmaintaining an equivalence ratio less than a threshold equivalence ratiobeing maintained during the transition initiation phase wherein saidthreshold equivalence ratio is an equivalence ratio at which NO_(x)production exceeds a corresponding threshold; the transition initiationphase comprising: decreasing air supply to the engine; decreasing anamount of hydrogen supplied to the engine; and initiating supply of aliquid fuel to the engine; and the transition completion phasecomprising: abruptly increasing supply of the liquid fuel; anddecreasing supply of hydrogen continually through the transitioncompletion phase.
 18. The method of claim 17 wherein said transition isinitiated based on a demand for increased BMEP that causes said hydrogenequivalence ratio to exceed about 0.5.
 19. The method of claim 17wherein the equivalence ratio threshold is about 0.85.
 20. The method ofclaim 17 wherein said transition is initiated in response to BMEP demandexceeding a threshold.
 21. A method for controlling an internalcombustion engine during transitions between hydrogen fuel and ahydrocarbon fuel, the method comprising: supplying hydrocarbon fuel tothe engine in an amount to produce an equivalence ratio that is eitherbelow a first threshold equivalence ratio or above a second equivalenceratio.
 22. The method of claim 21 wherein the first and secondequivalence ratios correspond to equivalence ratios associated withNO_(x) production exceeding a predetermined NO_(x) threshold.
 23. Themethod of claim 21 wherein the first equivalence ratio threshold isabout 0.85 and the second equivalence ratio threshold is 1.0.